What is required to break a partial vacuum when taking apart a positive displacement pump?

To effectively break a partial vacuum when disassembling a positive displacement pump, removing a drain plug from the casing is the appropriate action. This procedure allows air to enter the pump casing, equalizing the pressure both inside and outside the pump. When a positive displacement pump operates, it creates a partial vacuum on the suction side that can impede disassembly. By facilitating airflow through the casing, the risk of creating a vacuum that could lead to a difficult disassembly is minimized.

In contrast, the other actions would not adequately relieve the vacuum. Opening a discharge valve would help to relieve pressure on the discharge side but may not directly affect the suction side where the partial vacuum is present. Closing off the suction would only exacerbate the vacuum situation, making it harder to disassemble the pump. Priming the pump involves filling it with liquid to remove air, which is counterproductive when the intent is to take apart the pump. Therefore, removing the drain plug is the most effective method for safely breaking the vacuum when accessing the pump components.